1 Introduction

The term space weather refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and that can affect human life and health (definition used by the U.S. National Space Weather Plan). Of course, this definition also encompasses the generous energy supply from the Sun through its radiation that allows the existence of life on the Earth. However, this article is not meant to address this particular topic, except for the variability of radiation effects on very short time scales, e.g., in flares. Longer time scales such as decades or even centuries would be covered in Living Reviews in Solar Physics by a separate article under the title “Sun and Climate.”

Our modern hi-tech society has become increasingly vulnerable to disturbances from outside the Earth system, in particular to those initiated by explosive events on the Sun:

1. Flares release flashes of radiation covering an immense wavelength range (from radio waves to Gamma-rays) that can, e.g., heat up the terrestrial atmosphere within minutes such that satellites drop into lower orbits.
2. Solar energetic particles (SEPs), accelerated to near-relativistic energies during major solar storms arrive at the Earth’s orbit within minutes and may, among other things, severely endanger astronauts traveling through interplanetary space, i.e., outside the Earth’s protective magnetosphere.
3. Coronal mass ejections (CMEs), ejected into interplanetary space as gigantic clouds of ionized gas, that after a few hours or days may eventually hit the Earth and cause, among other effects, geomagnetic storms.

The economic consequences of these effects are enormous (see, e.g. Siscoe (2000); Lanzerotti (2001); Baker (2004), see also further articles in the books by Song et al. (2001) and Daglis et al. (2004)). That’s one reason why space weather and its predictability have recently attained major attention, not only with the involved scientists but also with the general public. Another reason is the new quality of observational data that have been obtained over the last decade from a new generation of space-based instruments. A huge fleet of spacecraft (ULYSSES, SOHO, YOKHOH, WIND, ACE, TRACE, RHESSI) has allowed us to advance our understanding of the processes involved near the Sun, in interplanetary space, and in the near-Earth environment, and thus to renew our picture of the Sun, the heliosphere, and the solar-terrestrial relationships (see, e.g., the review by Crooker, 2000).

For setting the stage for this article, I present a series of observations of the famous “Halloween events” that occurred during several days in late October/early November 2003. All important aspects of the space weather issue are addressed here in a very impressive way. The animation in Figure 1 shows a sequence of images taken with the EIT telescope on SOHO. A few very active regions moved across the Earth-facing side of the Sun and produced several bright flares and massive eruptions. Some of them resulted in powerful CMEs (see Figure 2 and 3 which are series of coronagraph images taken by the LASCO C2 and C3 instruments on SOHO), that were pointed towards the Earth and caused major geomagnetic storms. This type of CME where the brightening occurs simultaneously all around the coronagraphs occulting disk are called halo CMEs (Howard et al., 1982).

Intense fluxes of SEPs with relativistic energies were also generated, capable enough to penetrate the skins of spacecraft and instruments and even damage some. The “snow showers” in the images of Figures 1, 2 and 3 were in fact caused by such particles. Fortunately, the CCD cameras in these telescopes recovered after some hours. When it was finally realized how high that the radiation dose from such giant events can actually be, this issue became a primary concern in manned space exploration. Adequate protective measures must be found to ensure the astronauts’ safety on their future journeys to Moon and Mars (see, e.g. Wilson et al. (2004) and references therein).

The Halloween series of X-ray flares, SEP fluxes, interplanetary magnetic field (IMF) and geomagnetic index variations is illustrated in Figure 4. The X28 flare on November 4 was in fact the strongest solar X-ray flare since the beginning of regular recordings in 1968. The X-ray sensors on the GOES satellites even went into saturation. After some proper reconstruction using calibrated proxy data the real magnitude of this flare was determined X40 which means a peak flux of 4 mW∕m² at Earth (Woods et al., 2004; Brodrick et al., 2005). Further, on October 30, two of the 12 strongest geomagnetic storms (compare Cliver and Svalgaard (2004) since the beginning of D_st recording in 1932 occurred (−363 nT and −401 nT), with most dramatic consequences all over the globe. These storms were set loose right at those moments when the IMF turned southward (strong Bz south components).

Figure 4: The Halloween series (from October 28 to November 6, 2003) of X-ray flares (upper 2 panels), interplanetary magnetic field (inserted panel; the red curve is the Bz component), SEP fluxes (next 3 panels), and geomagnetic index (Kp) variations (bottom panel). The data were assembled from the NOAA webpages starting from http://www.sec.noaa.gov/today.html and the ACE page at http://sec.noaa.gov/ace/ACErtsw_home.html.

Detailed analyses of these extraordinary Halloween events and their effects were assembled in special editions of Geophysical Research Letters and Journal of Geophysical Research (see http://www.agu.org/journals/ss/VIOLCONN1/ and were reviewed by Veselovsky et al. (2004), (see also Gopalswamy et al., 2005b). These events demonstrate most impressively what space weather is about, with respect to both: its origin at the Sun, and its various effects on the Earth system.

Forecasting space weather effects is still a major challenge (Singer et al., 2001; Schwenn et al., 2005). The trustworthiness and accuracy in forecasting even the big solar events, i.e., flares and CMEs, and their impacts are still poor. They occur rather spontaneously, and we have not yet identified unique signatures that would indicate an imminent explosion and its probable onset time, location, strength, and significance for the Earth. The underlying physics is not sufficiently well understood, and thus we do not have appropriate warning tools at hands.

In this review, I will describe the several chains of actions originating in our parent star, the Sun, that affect Earth, with particular attention to the solar phenomena and the subsequent effects in interplanetary space. At first, we will inspect the solar wind itself: it is the medium in which the Earth system is imbedded and which determines the “ground state” of space weather. The solar wind interacts with the Earth’s intrinsic magnetic field and thus shapes the magnetosphere. By its variability the solar wind constantly moulds and remodels the magnetosphere. Finally, the solar wind is the medium through which disturbances from the Sun have to propagate.

Once a disturbance has reached the outer boundaries of the Earth system, a whole new series of processes will be triggered that are controlled by the Earth’s magnetic field, its ionosphere and atmosphere. Living Reviews in Solar Physics will cover this issue in a separate article “Space Weather: The Terrestrial Perspective.”

"Space Weather: The Solar Perspective" Rainer Schwenn http://www.livingreviews.org/lrsp-2006-2

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